This invention relates to a method of detecting abnormality in fuel supply systems of internal combustion engines, and more particularly to a method of detecting an abnormality occurring in a fuel supply system of an internal combustion engine on the basis of a learned average value of an air-fuel ratio correction coefficient which is determined in response to an output signal from an exhaust gas component concentration sensor used for air-fuel ratio feedback control of the engine.
Conventionally, a method of detecting abnormality in a fuel supply system of an internal combustion engine is known e.g. from Japanese Provisional Patent Publication (Kokai) No. 54-5129, in which when the engine is operating in an air-fuel ratio feedback control region, the air-fuel ratio of a mixture supplied to the engine is controlled by means of an air-fuel ratio correction coefficient which is determined in response to an output signal from an exhaust gas component concentration sensor arranged in the exhaust system of the engine, and at the same time an average value of the air-fuel ratio correction coefficient is calculated, whereby it is determined that an abnormality exists in the fuel supply system when the average value exceeds a predetermined reference range.
According to the above method, the average value K.sub.REF is learned based on the following equation: EQU K.sub.REF =K.sub.02 .times.(C/A)+K.sub.REF '.times.(A-C)/A
where K.sub.02 represents a value of the air-fuel ratio correction coefficient assumed upon inversion of the output level of the exhaust gas component concentration sensor or upon generation of each TDC signal pulse, K.sub.REF, an immediately preceding value of the learned average value K.sub.REF, A a constant, and C a variable which is set to a suitable value within a range of 1 to A.
The learned average value K.sub.REF is used for detecting abnormality in the fuel supply system, such as clogging of a fuel injection valve, lodging of a foreign matter in same, and aging of the system to such an extent that the fuel supply amount can no longer be properly controlled thereby. In order to detect such an abnormality promptly, the speed at which the air-fuel ratio correction coefficient K.sub.02 is learned has to be increased by setting the variable C to a value nearer to the constant A to thereby cause the learned average value K.sub.REF to more rapidly reflect changes in the value of the air-fuel ratio correction coefficient K.sub.02. However, if the variable C is set to a value near the constant A, the learned average value K.sub.REF reflect even an abnormal value of the air-fuel correction coefficient K.sub.02 which is temporarily assumed due to noise in the output signal from the sensor or the like, which may lead to a false detection of an abnormality in the fuel supply system. On the other hand, in order to detect an abnormality due to aging of the system, the variable C has to be set to a value nearer to 1 to thereby calculate a learned average value K.sub.REF free from temporary changes in the air-fuel ratio correction coefficient K.sub.02. However, in this case, the learned average value K.sub.REF too slowly reflects changes in the value of the air-fuel ratio correction coefficient K.sub.02, which results in a delayed detection of an abnormality in the fuel supply system.